Method and composition to enhance wetting of ECP electrolyte to copper seed

ABSTRACT

A composition and method which substantially enhances the wetting of an electrolyte solution to a surface in the electrochemical plating of a metal such as copper on the surface. The composition is an organic mixture which includes an organic acid, such as citric acid or acetic acid, and a low molecular weight ionic polymer such as an alcohol, an amine or alkyphenol alkoxylate. The method includes suspending the composition as a layer in the solution and passing the surface through the composition suspension layer to define a wetting layer on the surface. Consequently, metal electroplated onto the surface is substantially devoid of pits or other structural defects.

FIELD OF THE INVENTION

The present invention relates to electrochemical plating (ECP) processesused to deposit metal layers on semiconductor wafer substrates in thefabrication of semiconductor integrated circuits. More particularly, thepresent invention relates to a composition and method for enhancingwetting of electrochemical plating electrolyte to a metal seed layer inelectrochemical plating of metals, particularly copper, on a substrate.

BACKGROUND OF THE INVENTION

In the fabrication of semiconductor integrated circuits, metal conductorlines are used to interconnect the multiple components in devicecircuits on a semiconductor wafer. A general process used in thedeposition of metal conductor line patterns on semiconductor wafersincludes deposition of a conducting layer on the silicon wafersubstrate; formation of a photoresist or other mask such as titaniumoxide or silicon oxide, in the form of the desired metal conductor linepattern, using standard lithographic techniques; subjecting the wafersubstrate to a dry etching process to remove the conducting layer fromthe areas not covered by the mask, thereby leaving the metal layer inthe form of the masked conductor line pattern; and removing the masklayer typically using reactive plasma and chlorine gas, thereby exposingthe top surface of the metal conductor lines. Typically, multiplealternating layers of electrically conductive and insulative materialsare sequentially deposited on the wafer substrate, and conductive layersat different levels on the wafer may be electrically connected to eachother by etching vias, or openings, in the insulative layers and fillingthe vias using aluminum, tungsten or other metal to establish electricalconnection between the conductive layers.

Deposition of conductive layers on the wafer substrate can be carriedout using any of a variety of techniques. These include oxidation, LPCVD(low-pressure chemical vapor deposition), APCVD (atmospheric-pressurechemical vapor deposition), and PECVD (plasma-enhanced chemical vapordeposition). In general, chemical vapor deposition involves reactingvapor-phase chemicals that contain the required deposition constituentswith each other to form a nonvolatile film on the wafer substrate.Chemical vapor deposition is the most widely-used method of depositingfilms on wafer substrates in the fabrication of integrated circuits onthe substrates.

Due to the ever-decreasing size of semiconductor components and theever-increasing density of integrated circuits on a wafer, thecomplexity of interconnecting the components in the circuits requiresthat the fabrication processes used to define the metal conductor lineinterconnect patterns be subjected to precise dimensional control.Advances in lithography and masking techniques and dry etchingprocesses, such as RIE (Reactive Ion Etching) and other plasma etchingprocesses, allow production of conducting patterns with widths andspacings in the submicron range. Electrodeposition or electroplating ofmetals on wafer substrates has recently been identified as a promisingtechnique for depositing conductive layers on the substrates in themanufacture of integrated circuits and flat panel displays. Suchelectrodeposition processes have been used to achieve deposition of thecopper or other metal layer with a smooth, level or uniform top surface.Consequently, much effort is currently focused on the design ofelectroplating hardware and chemistry to achieve high-quality films orlayers which are uniform across the entire surface of the substrates andwhich are capable of filling or conforming to very small devicefeatures. Copper has been found to be particularly advantageous as anelectroplating metal.

Electroplated copper provides several advantages over electroplatedaluminum when used in integrated circuit (IC) applications. Copper isless electrically resistive than aluminum and is thus capable of higherfrequencies of operation. Furthermore, copper is more resistant toelectromigration (EM) than is aluminum. This provides an overallenhancement in the reliability of semiconductor devices because circuitswhich have higher current densities and/or lower resistance to EM have atendency to develop voids or open circuits in their metallicinterconnects. These voids or open circuits may cause device failure orburn-in.

A typical standard or conventional electroplating system for depositinga metal such as copper onto a semiconductor wafer includes a standardelectroplating cell having an adjustable current source, a bathcontainer which holds an electrolyte solution (typically acid coppersulfate solution), and a copper anode and a cathode immersed in theelectrolyte solution. The cathode is the semiconductor wafer that is tobe electroplated with metal. Both the anode and the semiconductorwafer/cathode are connected to the current source by means of suitablewiring. The electrolyte solution may include an additive for filling ofsubmicron features and leveling the surface of the copper electroplatedon the wafer. An electrolyte holding tank may further be connected tothe bath container for the addition of extra electrolyte solution to thebath container.

In operation of the electroplating system, the current source applies aselected voltage potential typically at room temperature between theanode and the cathode/wafer. This potential creates a magnetic fieldaround the anode and the cathode/wafer, which magnetic field affects thedistribution of the copper ions in the bath. In a typical copperelectroplating application, a voltage potential of about 2 volts may beapplied for about 2 minutes, and a current of about 4.5 amps flowsbetween the anode and the cathode/wafer. Consequently, copper isoxidized at the anode as electrons from the copper anode and reduce theionic copper in the copper sulfate solution bath to form a copperelectroplate at the interface between the cathode/wafer and the coppersulfate bath.

The copper oxidation reaction which takes place at the anode isillustrated by the following reaction equation:Cu---->Cu⁺⁺+2e ⁻

The oxidized copper cation reaction product forms ionic copper sulfatein solution with the sulfate anion in the bath 20:Cu⁺⁺+SO₄ ⁻⁻---->Cu⁺⁺SO₄ ⁻⁻

At the cathode/wafer, the electrons harvested from the anode flowedthrough the wiring reduce copper cations in solution in the coppersulfate bath to electroplate the reduced copper onto the cathode/wafer:Cu⁺⁺+2e ⁻---->Cu

When a copper layer is deposited on a substrate, such as byelectrochemical plating, the copper layer must be deposited on a metalseed layer such as copper which is deposited on the substrate prior tothe copper ECP process. Seed layers may be applied to the substrateusing any of a variety of methods, such as by physical vapor deposition(PVD) and chemical vapor deposition (PVD). Typically, metal seed layersare thin (about 50-1500 angstroms thick) in comparison to conductivemetal layers deposited on a semiconductor wafer substrate.

Metal seed layers deposited on a substrate may suffer from variousproblems such as the presence of metal oxide on the seed layer anddiscontinuities in the layer, as well as contamination and the formationof pits in the layer. These drawbacks cause non-uniform wetting of theelectroplating electrolyte solution to the seed layer surface.Non-uniform wetting of the electrolyte solution to the seed layer causesstructural defects such as pits in the metal electroplated onto the seedlayer, compromising the structural and functional integrity of thefinished IC devices fabricated on the substrate.

Traditional approaches to improving the wetting of an electroplatingelectrolyte solution to a metal seed layer include pre-rinsing orpre-annealing of the seed layer surface. However, both of these methodsachieve unsatisfactory results. Accordingly, a new and improvedcomposition and method is needed for enhancing the wetting of anelectroplating electrolyte solution to a metal seed layer in theelectrochemical plating of copper or other metal on a substrate.

An object of the present invention is to provide a novel composition andmethod for the pre-treatment or wetting of a seed layer on a substrateprior to electroplating a metal on the seed layer.

Another object of the present invention is to provide a novelcomposition and method for enhancing the wetting of an electrolytesolution to a metal seed layer in the electrochemical plating of copperor other metal on a substrate.

Still another object of the present invention is to provide a novelcomposition and method which results in electroplating of a metal layersubstantially devoid of structural defects on a seed layer provided on asubstrate and improves gap fill capability through wetting improvement.

Yet another object of the present invention is to provide a novelcomposition and method which substantially reduces the contact angle ofan electrolyte solution on a seed layer in the electrochemical platingof copper or other metal on the seed layer.

SUMMARY OF THE INVENTION

In accordance with these and other objects and advantages, the presentinvention is generally directed to a composition and method whichsubstantially enhances the wetting of an electrolyte solution to a seedlayer on a substrate in the electrochemical plating of a metal such ascopper on the seed layer. The composition is an organic mixture whichincludes an organic acid, such as citric acid or acetic acid, and a lowmolecular weight ionic polymer such as an alcohol, an amine oralkyphenol alkoxylate. According to a typical method of the invention, ametal seed layer is initally deposited on the substrate. Anelectrochemical plating (ECP) electrolyte solution is prepared, and theorganic composition mixture is dispensed as a layered suspension intothe solution. The substrate, with the metal seed layer depositedthereon, is then moved through the suspended composition mixture layerand into the ECP electrolyte solution, such that some of the compositionis layered into a wetting layer on the seed layer and enhances wettingof the electrolyte solution to the metal seed layer on the substrate.The substrate is then suspended in the solution and subjected toelectrochemical plating. The electroplated metal forms a layer of highstructural integrity substantially devoid of pits or other structuraldefects across the entire surface of the seed layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example, with referenceto the accompanying drawings, in which:

FIG. 1 is a schematic of an electrochemical plating system inimplementation of the present invention;

FIG. 1A is a side view of a substrate being moved through a compositionmixture layer suspended in an electroplating electrolyte bath, to form awetting layer of the composition mixture on a seed layer provided on thesubstrate;

FIG. 2 is a flow diagram illustrating a typical sequential flow ofprocess steps in implementation of the method of the present invention;and

FIG. 3 is a graph, with contact angle (on the Y-axis) plotted vs.electroplating process Q-time (on the X-axis), illustrating the contactangle of electrolyte solution on a seed layer treated according to themethod of the present invention, compared to the contact angle ofelectrolyte solution on an untreated seed layer.

DETAILED DESCRIPTION OF THE INVENTION

The present invention has particularly beneficial utility in the wettingof an electrolyte solution to a copper seed layer on a semiconductorwafer substrate to enhance the structural quality of a copper layerelectroplated onto the seed layer in the fabrication of semiconductorintegrated circuits. However, the invention is more generally applicableto the wetting of electroplating electrolyte solution to metal seedlayers other than copper, and is adaptable to the electroplating ofmetals including but not limited to copper on a substrate in a varietyof industrial applications.

The present invention is generally directed to a composition and methodfor substantially enhancing the wetting of an electrolyte solution on aseed layer deposited on a substrate for the subsequent electrochemicalplating (ECP) deposition of a metal, particularly copper, on the seedlayer. As compared to untreated control seed layers, the compositionsignificantly reduces the contact angle of electrolyte solution on theseed layer. Consequently, the metal electroplated onto the seed layer issubstantially devoid of pits and other structural defects across theentire surface of the seed layer.

The composition of the present invention includes a mixture of anorganic acid, such as citric acid or acetic acid, and a non-ionicpolymer such as an alcohol, an amine or an alkyphenol alkoxylate.Preferably, the non-ionic polymer is a low molecular weight (<1,000 MW)non-ionic polymer. The organic acid is present in the compositionmixture in a quantity of from typically about 2 to about 20 wt. %. Thenon-ionic polymer is present in the composition mixture in a quantity offrom typically about 0.5 to about 10 wt. %.

In one embodiment, the composition includes a mixture of an organic acidsuch as citric acid or acetic acid and an alkoxylated alcohol such asethoxylated alcohol polymer. Preferably, the composition includestypically about 10 wt. % of the organic acid and typically about 5 wt. %of the alkoxylated alcohol.

In another embodiment, the composition includes a mixture of an organicacid such as citric acid or acetic acid and an alkoxylated polymer aminesuch as ethoxylated diamine polymer. Preferably, the compositionincludes typically about 10 wt. % of the organic acid and typicallyabout 5 wt. % of the amine.

In still another embodiment, the composition includes a mixture of anorganic acid such as citric acid or acetic acid and alkyphenolalkoxylate. Preferably, the composition includes typically about 10 wt.% of the organic acid and typically about 5 wt. % of the alkyphenolalkoxylate.

Referring to FIG. 1, an electrochemical plating (ECP) system 10 suitablefor implementation of the present invention includes a standardelectroplating cell having an adjustable current source 12, a bathcontainer 14, a copper anode 16 and a cathode 18, which cathode 18 isthe semiconductor wafer substrate that is to be electroplated withcopper. The anode 16 and cathode/substrate 18 are connected to thecurrent source 12 by means of suitable wiring 38. The bath container 14holds a bath 20 typically of acid copper sulfate solution which mayinclude an additive for filling of submicron features and leveling thesurface of the copper electroplated on the substrate 18.

The ECP system 10 may further include a pair of bypass filter conduits24, a bypass pump/filter 30, and an electrolyte holding tank 34 for theintroduction of additional electrolytes into the bath container 14, asnecessary. The bypass filter conduits 24 extend through the anode 16 andopen to the upper, oxidizing surface 22 of the anode 16 at opposite endsof the anode 16. The bypass filter conduits 24 connect to the bypasspump/filter 30 located outside the bath container 14, and the bypasspump/filter 30 is further connected to the electrolyte holding tank 34through a tank inlet line 32. The electrolyte holding tank 34 is, inturn, connected to the bath container 14 through a tank outlet line 36.It is understood that the ECP system 10 heretofore described representsjust one example of a possible system which is suitable forimplementation of the present invention, and other systems ofalternative design may be used instead.

The process of the invention may be used with any formulation for theelectroplating bath solution 20, such as copper, aluminum, nickel,chromium, zinc, tin, gold, silver, lead and cadmium electroplatingbaths. The present invention is also suitable for use withelectroplating baths containing mixtures of metals to be plated onto asubstrate. It is preferred that the electroplating bath 20 be a copperalloy electroplating bath, and more preferably, a copper electroplatingbath. Typical copper electroplating bath formulations are well known tothose skilled in the art and include, but are not limited to, anelectrolyte and one or more sources of copper ions. Suitableelectrolytes include, but are not limited to, sulfuric acid, aceticacid, fluoroboric acid, methane sulfonic acid, ethane sulfonic acid,trifluormethane sulfonic acid, phenyl sulfonic acid, methyl sulfonicacid, p-toluenesulfonic acid, hydrochloric acid, phosphoric acid and thelike. The acids are typically present in the bath in a concentration inthe range of from about 1 to about 300 g/L. The acids may furtherinclude a source of halide ions such as chloride ions. Suitable sourcesof copper ions include, but are not limited to, copper sulfate, copperchloride, copper acetate, copper nitrate, copper fluoroborate, coppermethane sulfonate, copper phenyl sulfonate and copper p-toluenesulfonate. Such copper ion sources are typically present in aconcentration in the range of from about 10 to about 300 g/L ofelectroplating solution.

Referring to FIGS. 1, 1A and 2, according to the method of the presentinvention, a metal seed layer 19, such as copper, is deposited on awafer substrate 18, as indicated in step S1 of FIG. 2. The metal seedlayer 19 may be deposited on the substrate 18 using conventionalchemical vapor deposition (CVD) or physical vapor deposition (PVD)techniques, according to the knowledge of those skilled in the art. Theseed layer 19 has a thickness of typically about 50-1500 angstroms.

As indicated in step S2 of FIG. 2, the electrochemical plating (ECP)electrolyte bath solution 20 is prepared in the bath container 14. Next,as indicated in step S3, the organic composition mixture of the presentinvention is prepared and then suspended as a composition suspensionlayer 26 in the bath solution 20. The anode 16 and substrate 18 are thenimmersed in the bath solution 20 and connected to the adjustable currentsource 12 typically through wiring 38.

As shown in FIG. 1A and indicated in step S4 of FIG. 2, thecathode/substrate 18 is immersed in the bath solution 20 by passing thesubstrate 18 through the composition suspension layer 26. As shown inFIG. 1A, the seed layer 19 on the substrate 18 contacts the compositionsuspension layer 26 and causes a wetting layer 26 a to break off of thecomposition suspension layer 26 and adhere to the surface of the seedlayer 19. This wetting layer 26 a remains on the seed layer 19 duringthe subsequent electroplating process. It will be appreciated by thoseskilled in the art that the wetting layer 26 a promotes wetting of theECP electrolyte bath solution 20 to the seed layer 19 during theelectroplating process.

As indicated in step S5, a metal layer (not shown) is electroplated ontothe seed layer 19 as follows. In operation of the ECP system 10, thecurrent source 12 applies a selected voltage potential, typically atroom temperature, between the anode 16 and the cathode/substrate 18.This voltage potential creates a magnetic field around the anode 16 andthe cathode/substrate 18, which magnetic field affects the distributionof the copper ions in the bath solution 20. In a typical copperelectroplating application, a voltage potential of about 2 volts may beapplied for about 2 minutes, and a current of about 4.5 amps flowsbetween the anode 16 and the cathode/substrate 18. Consequently, copperis oxidized typically at the oxidizing surface 22 of the anode 16 aselectrons from the copper anode 16 reduce the ionic copper in the coppersulfate solution bath 20 to form a copper electroplate (not illustrated)at the interface between the cathode/substrate 18 and the copper sulfatebath 20. By promoting uniform wetting of the electrolyte bath solution20 to the entire surface of the seed layer 19, the wetting layer 26 afacilitates electroplating of the metal onto the seed layer 19 as acontinuous metal layer substantially devoid of structural deformitiessuch as pits. Accordingly, the electroplated metal layer on thesubstrate 18 contributes to the fabrication of IC devices that arecharacterized by high structural and operational integrity.

Referring next to the graph of FIG. 3, it is indicated that the contactangle of an ECP electrolyte bath solution on a seed layer treatedaccording to the composition and method of the present invention is lessthan 20%. This is compared to a contact angle of about 30˜35%, withrespect to control seed layers in which the seed layer is untreatedprior to the electroplating process. Consequently, the metalelectroplated onto the treated seed layer is substantially devoid ofpits and other structural defects which would otherwise reduce thequality of IC devices fabricated in the electroplated metal layer.

While the preferred embodiments of the invention have been describedabove, it will be recognized and understood that various modificationscan be made in the invention and the appended claims are intended tocover all such modifications which may fall within the spirit and scopeof the invention.

1. An electrolyte for copper electroplating, comprising: an electrolytesolution; and a composition comprising an organic acid and a non-ionicpolymer mixed with said organic acid provided in said electrolytesolution.
 2. The electrolyte of claim 1 wherein said organic acid iscitric acid or acetic acid.
 3. The electrolyte of claim 1 wherein saidnon-ionic polymer is an alcohol, an amine or alkyphenol alkoxylate. 4.The electrolyte of claim 1 wherein said composition is present in saidelectrolyte solution in a concentration of about 5% by weight.
 5. Theelectrolyte of claim 1 wherein said non-ionic polymer has a molecularweight of less than 1,000.
 6. The electrolyte of claim 5 wherein saidorganic acid is citric acid or acetic acid.
 7. The electrolyte of claim1 wherein said organic acid is present in said composition in a wt. % ofabout 10, and wherein said ionic polymer is present in said compositionin a wt. % of about
 5. 8. The electrolyte of claim 7 wherein saidorganic acid is citric acid or acetic acid and said non-ionic polymer isan alcohol, an amine or alkyphenol alkoxylate.
 9. An electrolyte forcopper electroplating, comprising: an electrolyte solution; and acomposition comprising an organic acid and a non-ionic polymer mixedwith said organic acid provided in a suspension layer in saidelectrolyte solution.
 10. The electrolyte of claim 9 wherein saidorganic acid is citric acid or acetic acid.
 11. The electrolyte of claim9 wherein said non-ionic polymer is an alcohol, an amine or alkyphenolalkoxylate.
 12. The electrolyte of claim 11 wherein said composition ispresent in said electrolyte solution in a concentration of about 5% byweight.
 13. The electrolyte of claim 9 wherein said organic acid ispresent in said composition in a wt. % of about 10, and wherein saidnon-ionic polymer is present in said composition in a wt. % of about 5.14. The electrolyte of claim 13 wherein said organic acid is citric acidor acetic acid.
 15. The electrolyte of claim 13 wherein said non-ionicpolymer is an alcohol, an amine or alkyphenol alkoxylate.
 16. Theelectrolyte of claim 15 wherein said organic acid is citric acid oracetic acid.
 17. A method for electroplating a metal onto a surface inan electroplating electrolyte solution, comprising the steps of:providing a composition mixture comprising an organic acid and anon-ionic polymer; forming a suspension layer of said compositionmixture in said solution; forming a wetting layer on said surface bypassing said surface through said suspension layer and into saidsolution; and electroplating said metal onto said surface.
 18. Themethod of claim 17 wherein said organic acid is citric acid or aceticacid and said non-ionic polymer is an alcohol, an amine or alkyphenolalkoxylate.
 19. The method of claim 17 wherein said organic acid ispresent in said composition in a wt. % of about 10, and wherein saidionic polymer is present in said composition in a wt. % of about
 5. 20.The method of claim 17 further comprising a substrate and wherein saidsurface comprises a metal seed layer deposited on said substrate.